A growing number of recombinant proteins are being developed for therapeutic, diagnostic, agricultural, veterinary, nutritional and other applications; however, many of these proteins may be difficult or expensive to produce in a functional form in the substantial quantities using conventional methods.
Conventional methods often involve inserting the gene responsible for the production of a particular protein into host cells such as bacteria, yeast, or mammalian cells. The cells are grown in culture medium and the desired protein is recovered from the cells or the culture medium. Traditional bacteria or yeast systems are sometimes unable to produce a complex protein in finctional form. While some mammalian cells can reproduce complex proteins, they are often difficult and expensive to grow, and produce only protein in relatively low amounts. In addition, non-secreted proteins are relatively difficult to purify from procaryotic or mammalian cells, as they are often not secreted into the culture medium.